Effect of ASE on the gain modulation in EDFAs

We investigate the effect of ASE on the gain modulation, referred to also as overmodulation. The gain modulation is the low-frequency ( kHz) amplitude modulation of the EDFA pump and the communication signal used for propagating line monitoring information. We develop the model of Novak and Moesle (2002), by including amplified spontaneous emission (ASE), that they neglected it. The derivation of an analytical model for EDFA overmodulation response has been presented. This model provide analytical expressions for the pump and input signal overmodulation responses, respectively. These expressions describe the output signal modulation index amplitude and phase, assuming small sinusoidal steady-state oscillations of the mean pump or input signal power. In this paper we show that ASE have some effect on predictions in the high gain/low saturation regime.

1. INTRODUCTION

In regard of electronic regeneration, direct optical amplification using erbium-doped fiber amplifiers offers many advantages for long haul repeatered transmission. First, the repeater can be quite simply configured regardless of the line signal bit rate. This feature becomes more significant as the line signal bit rate exceeding 1 Tb/s at which speed electronic regeneration requires high-speed electronic circuits, thereby resulting in increase in hardware cost and power consumption. Therefore, optical amplifiers are particularly useful in submarine repeaters that have severe space and power constraints. Second, optical amplifiers are flexible as regard bit rate and modulation format, and support wavelength division multiplexed signal transmission. In fact, their deployment in WDM systems after 1995 revolutionized the field of fiber-optic communications [1], [2].